These hydrophilic (and mostly large) hormone molecules bind to receptors on the surface of "target" cells; that is, cells able to respond to the presence of the hormone. These receptors are transmembrane proteins. Binding of the hormone to its receptor initiates a sequence of intracellular signals that may

Steroid hormones, being hydrophobic molecules, diffuse freely into all cells. However, their "target" cells contain cytoplasmic and/or nuclear proteins that serve as receptors of the hormone. The hormone binds to the receptor and the complex binds to hormone response elements — stretches of DNA within the promoters of genes responsive to the hormone. The hormone/receptor complex acts as a transcription factor turning target genes "on" (or "off").

Although a few hormones circulate simply dissolved in the blood, most are carried in the blood bound to plasma proteins. For example, all the steroid hormones, being highly hydrophobic, are transported bound to plasma proteins.